U.S. patent application number 15/620756 was filed with the patent office on 2018-02-01 for eddy current detection.
This patent application is currently assigned to Radiation Monitoring Devices, Inc.. The applicant listed for this patent is Radiation Monitoring Devices, Inc.. Invention is credited to Noa M. Rensing, Mark Steinback, Timothy C. Tiernan, Evan R. Weststrate.
Application Number | 20180031646 15/620756 |
Document ID | / |
Family ID | 45493093 |
Filed Date | 2018-02-01 |
United States Patent
Application |
20180031646 |
Kind Code |
A1 |
Tiernan; Timothy C. ; et
al. |
February 1, 2018 |
EDDY CURRENT DETECTION
Abstract
Eddy current detection probes and related methods are disclosed.
In some embodiments, the eddy current detection probes are hybrid
probes, including a solid state sensor and a detection loop. In
some embodiments, the eddy current detection probes include a drive
coil and a detection loop, with the detection loop having a
sensitive axis that is not parallel to principal axis of the drive
coil. In some such embodiments, the sensitive axis of the detection
loop is perpendicular to the principal axis of the drive coil.
Inventors: |
Tiernan; Timothy C.;
(Newton, MA) ; Steinback; Mark; (Newton, MA)
; Rensing; Noa M.; (West Newton, MA) ; Weststrate;
Evan R.; (Newton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Radiation Monitoring Devices, Inc. |
Watertown |
MA |
US |
|
|
Assignee: |
Radiation Monitoring Devices,
Inc.
Watertown
MA
|
Family ID: |
45493093 |
Appl. No.: |
15/620756 |
Filed: |
June 12, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13166729 |
Jun 22, 2011 |
9678175 |
|
|
15620756 |
|
|
|
|
61367648 |
Jul 26, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2201/00 20130101;
G01B 2210/00 20130101; G01N 1/00 20130101; G01N 27/9033 20130101;
G01B 1/00 20130101; G01R 1/00 20130101; G01R 33/09 20130101; G01R
33/096 20130101 |
International
Class: |
G01R 33/09 20060101
G01R033/09; G01N 27/90 20060101 G01N027/90 |
Claims
1. (canceled)
2. An eddy current detection probe, comprising: a drive coil; and a
conductive detection loop oriented substantially perpendicular to
the drive coil, wherein the conductive detection loop has a height
that is substantially perpendicular to the drive coil and less than
2.5 mm.
3. The eddy current detection probe of claim 2, wherein the drive
coil is contained at least substantially within a plane, and
wherein the plane is configured to be parallel to a surface of a
material specimen probed by the eddy current detection probe.
4. The eddy current detection probe of claim 2, wherein the
conductive detection loop has a side disposed in a same plane as
that in which the drive coil is disposed.
5. The eddy current detection probe of claim 2, wherein the drive
coil is configured to generate a drive magnetic field along a first
axis, and wherein the conductive detection loop has a detection
axis oriented substantially perpendicular to the first axis
6. The eddy current detection probe of claim 2, further comprising:
a substrate have a substantially planar surface; wherein the drive
coil is disposed on the substantially planar surface, and wherein
the conductive detection loop is wrapped at least partially around
the substrate.
7. The eddy current detection probe of claim 2, wherein the
conductive detection loop is formed at least partially by a first
electrical lead and a second electrical lead.
8. The eddy current detection probe of claim 2, wherein the
conductive detection loop encloses an area of less than
approximately 0.2 square millimeters.
9. The eddy current detection probe of claim 2, wherein the drive
coil is formed on a printed circuit board.
10. The eddy current detection probe of claim 9, wherein the
conductive detection loop is formed at least partially on the
printed circuit board.
11. A method, comprising: exciting a drive coil of an eddy current
detection probe with an alternating current (AC) signal having a
frequency greater than approximately 1 MHz; applying to a material
under test an incident magnetic field generated by the drive coil
as a result of exciting the drive coil; and detecting an induced
magnetic field from the material under test using a conductive
detection loop oriented substantially perpendicular to the drive
coil.
12. The method of claim 11, wherein the conductive detection loop
has a height that is substantially perpendicular to the drive coil
and less than 1 mm.
13. The method of claim 11, wherein the conductive detection loop
has a height that is substantially perpendicular to the drive coil
and less than 0.5 mm.
14. The method of claim 11, wherein exciting the drive coil with an
AC signal comprises exciting the drive coil with an AC signal
having a frequency between approximately 2 MHz and 5 MHz.
15. The method of claim 11, wherein exciting the drive coil with an
AC signal comprises exciting the drive coil with an AC signal
having a frequency greater than approximately 2 MHz.
16. The method of claim 11, wherein applying to the material under
test an incident magnetic field generated by the drive coil as a
result of exciting the drive coil comprises orienting the drive
coil substantially parallel to a surface of the material under
test.
17. The method of claim 16, wherein exciting the drive coil with an
AC signal comprises exciting the drive coil with an AC signal
having a frequency between approximately 2 MHz and 5 MHz.
18. The method of claim 17, wherein the conductive detection loop
has a height that is substantially perpendicular to the drive coil
and less than 2.5 mm.
19. The method of claim 11, wherein the detected signal is
preconditioned by applying a bucking signal, comprising an inverted
reference signal at the drive frequency.
20. The method of claim 19, wherein the reference signal is
obtained by making a reference measurement at a point of an article
to be tested known to be free of defects.
21. A method of manufacturing an eddy current detection probe,
comprising: fabricating a conductive drive coil on a first layer of
a multi-layered substrate; fabricating conductive traces on at
least two layers of the multi-layered substrate; and
interconnecting with conductive material the conductive traces on
the at least two layers of the multi-layered substrate to form a
detection loop substantially perpendicular to the conductive drive
coil.
Description
RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 13/166,729, filed Jun. 22, 2011, which claims
the benefit under 35 U.S.C. 119(e) to U.S. Provisional Patent
Application Ser. No. 61/367,648, filed Jul. 26, 2010, which are
incorporated herein by reference in their entirety.
BACKGROUND
Field
[0002] The present application discloses various aspects relating
to eddy current detection technology.
Related Art
[0003] In general, materials may have defects (or flaws) in them,
such as cracks, inclusions and corrosion. The defects may form for
various reasons, including as a result of manufacturing and/or
stresses experienced by the material over its lifetime.
[0004] One manner for detecting such defects in a conductive
material (such as a metal or metal alloy) is to generate eddy
currents within the material and detect the resulting magnetic
fields. Eddy currents are generated in a conductive material in
response to a suitable time varying magnetic field being applied to
the conductive material. The time varying magnetic field gives rise
to a force on the electrons in the conductive material, thus
creating current, referred to as "eddy current." The eddy currents
themselves give rise to magnetic fields, referred to as induced
magnetic fields, which oppose the incident magnetic field. The
distributions of the eddy currents will be altered by cracks (or
other defects) in the material, thus creating perturbations in the
induced magnetic fields. The changes in the induced magnetic
fields, which are detected with an eddy current probe, give an
indication of the presence of the cracks (or other defects) and
their characteristics (e.g., location, size, shape, etc.).
Generally, the magnetic field due to the coil as well as the
magnetic fields arising from the eddy currents induced in a uniform
material have a well characterized spatial distribution which is
exactly axial at the center of the current loop and has field lines
that surround the current distribution. The magnetic fields has
both radial and axial components, and near the center the in plane
component is very small. For a circular coil and a uniform
material, the tangential component of both direct and induced
magnetic fields is zero. In contrast, if there are cracks or other
irregularities in the material which disrupt the eddy currents and
perturb the magnetic field, the induced magnetic field may be
modified and may have substantial in-plane components and possibly
substantial tangential components. These in-plane components may be
easier to detect than changes in the substantial axial magnetic
field.
[0005] This effect may be especially pronounced if the position of
the crack or flaw relative to the coil is such that the maximum
eddy current density would pass through that point in the absence
of the flaw, and if the characteristic depth of the eddy current
distribution is comparable to or smaller than the extent of the
crack or flaw in the depth direction.
[0006] Conventional coil-based eddy current probes generally take
one of two forms. A first type of conventional coil-based eddy
current probe uses a single coil (i.e., a combined drive/detection
coil) to both carry the current that generates (or drives) the
incident magnetic field applied to the conductive material under
test and detect the magnetic field due to the eddy currents in the
material under test. Monitoring this field allows the instrument to
detect changes caused by cracks or other flaws. A second type of
conventional coil-based eddy current probe uses two distinct
co-axial coils--one which carries the current that generates (or
drives) the incident magnetic field applied to the conductive
material under test and a second which detects the total magnetic
field and can be monitored to detect changes due to cracks (or
other defects) in the material under test.
[0007] FIG. 1 illustrates a conventional coil-based eddy current
probe of the first type. The probe 100 includes a single coil 102
through which an alternating current (AC) current is applied to
generate a magnetic field incident upon a conductive material under
test 104 when the probe is placed in proximity to the material
under test. The incident magnetic field gives rise to eddy currents
in the material under test 104 as shown which generate a magnetic
flux which passes through the coil 102. A crack (or other type of
defect) 106 in the material under test 104 disturbs the eddy
currents 108 and therefore the magnetic flux. The disturbance in
the magnetic flux thus indicates the presence of the crack (or
other type of defect).
[0008] FIG. 2 illustrates a conventional coil-based eddy current
probe of the second type. As shown, the probe 200 includes two
distinct but co-axial coils, a drive coil 202 to generate the eddy
currents in the material under test by applying an incident
magnetic field and a detection coil 204 (of one or more turns) to
detect the magnetic flux resulting from the eddy current. Because
the coils 202 and 204 are co-axial, the sensitive axis of the
detection coil is parallel to the principal axis of the drive coil
(i.e., the primary direction of the magnetic field generated by the
drive coil).
[0009] It should be appreciated from FIGS. 1 and 2 that both of
these types of conventional coil-based eddy current probes use a
detection coil that is sensitive to the magnetic field components
oriented in the same direction as the magnetic fields created by
the drive coil. These fields are generally oriented in the
direction normal to the surface of the material under test.
[0010] In the case of a two coil eddy current sensor, however, the
detection coil may alternately be arranged with its axis at an
angle to the drive coil axis, so as to be more sensitive to in
plane components of the magnetic field or specifically to the
tangential direction or to in-plane components (either tangential
or radial) at the center of the coil, or to reduce its sensitivity
to the out-of-plane component of the magnetic field.
[0011] Some conventional eddy current probes do not use a detection
coil, and instead use a solid-state magnetic field detecting
element. These include magneto-resistive sensors (such as
anisotropic (AMR) or giant magnetoresistive sensors), Hall Effect
sensors, and superconducting quantum interference devices (SQUIDS).
In the case of magnetoresistive sensors, the resistance of the
sensor varies depending on the magnetic field applied to the
sensor. Thus, when an AMR sensor is placed in the presence of an
eddy current, the magnetic fields generated by the eddy current may
alter the resistance value of the AMR sensor. The alteration in the
resistance value is used to detect the presence and strength of the
eddy currents and thus of any defects in the material under
test.
SUMMARY
[0012] Eddy current detection probes and related methods are
disclosed.
[0013] In one aspect, an eddy current detection probe is provided.
The probe includes a solid state sensor configured to sense changes
in a magnetic field created by perturbations in an eddy current.
The probe further includes a conductive detection loop configured
to receive the magnetic field and generate a voltage in response to
variation in the magnetic field.
[0014] In one aspect, an eddy current detection probe is provided.
The probe includes a substrate having a substantially planar
surface and a drive coil disposed on the substantially planar
surface. The probe further includes an anisotropic magnetoresistive
(AMR) sensor disposed on the substantially planar surface of the
substrate and positioned within a perimeter of the drive coil. The
probe further includes a detection loop comprising a first
electrical lead and a second electrical lead. The first electrical
lead is connected to a first end of the AMR sensor and the second
electrical lead is connected to a second end of the AMR sensor. The
first electrical lead and the second electrical lead are positioned
at least partially around the substrate to form the detection loop.
The detection loop is substantially perpendicular to the drive
coil.
[0015] In one aspect, an eddy current probe is provided. The probe
includes a first substrate having a substantially planar surface
and a drive coil disposed on the substantially planar surface of
the first substrate. The probe includes a second substrate smaller
than the first substrate. The second substrate has a substantially
planar surface. The probe includes an anisotropic magnetoresistive
(AMR) sensor disposed on the substantially planar surface of the
second substrate and attached to the first substrate within a
perimeter of the drive coil. The probe further includes a detection
loop comprising a first electrical lead and a second electrical
lead. The first electrical lead is connected to a first end of the
AMR sensor and the second electrical lead is connected to a second
end of the AMR sensor. The first electrical lead and second
electrical lead are positioned at least partially around the first
substrate to form the detection loop. The detection loop is
substantially perpendicular to the drive coil.
[0016] In one aspect, an eddy current detection probe is provided.
The probe includes a drive coil and a conductive detection loop
oriented substantially perpendicular to the drive coil. The
conductive detection loop has a height that is substantially
perpendicular to the drive coil and less than 2.5 mm. In some
embodiments, the conductive detection loop has a height that is
less than 0.5 mm.
[0017] In one aspect, a method is provided. The method comprises
exciting a drive coil of an eddy current detection probe with an
alternating current (AC) signal having a frequency greater than
approximately 1 MHz and applying to a material under test an
incident magnetic field generated by the drive coil as a result of
exciting the drive coil. The method further comprises detecting an
induced magnetic field from the material under test using a
conductive detection loop oriented substantially perpendicular to
the drive coil.
[0018] In one aspect, a method of manufacturing an eddy current
probe is provided. The method comprises fabricating a conductive
drive coil on a first layer of a multi-layered substrate; and
fabricating conductive traces on at least two layers of the
multi-layered substrate. The method further comprises
interconnecting with conductive material the conductive traces on
the at least two layers of the multi-layered substrate to form a
detection loop substantially perpendicular to the conductive drive
coil.
[0019] In one aspect, a probe for depth profiling is provided. The
probe comprises a drive coil configured to be positioned proximate
a surface of an article being tested such that an axis of the drive
coil is perpendicular to the surface of the article. The probe
further comprises a magnetoresistive sensor constructed and aligned
to respond to electrical conductivity and/or magnetic permeability
differences between material in a layer at the surface of the
article and material in the interior of the article.
[0020] Other aspects, embodiments and features of the invention
will become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying
drawings. The accompanying figures are schematic and are not
intended to be drawn to scale. In the figures, each identical, or
substantially similar component that is illustrated in various
figures is represented by a single numeral or notation. For
purposes of clarity, not every component is labeled in every
figure. Nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 illustrates a first type of conventional coil-based
eddy current probe having a combined drive/detection coil.
[0022] FIG. 2 illustrates a second type of conventional coil-based
eddy current probe having distinct, co-axial drive and detection
coils.
[0023] FIG. 3 is a non-limiting example of an eddy current
detection probe including a solid state sensor and a conducting
loop, according to one non-limiting embodiment.
[0024] FIG. 4 illustrates an example of four different locations of
a solid state sensor relative to a drive coil of an eddy current
detection probe according to one non-limiting embodiment.
[0025] FIG. 5 illustrates an eddy current detection probe including
a drive coil and a detection loop according to another non-limiting
embodiment.
[0026] FIG. 6 illustrates an eddy current detection probe including
a drive coil and a detection loop which surrounds the conductor of
the drive coil.
[0027] FIG. 7 illustrates an eddy current detection probe including
a drive coil and a detection loop according to another non-limiting
embodiment.
[0028] FIG. 8 illustrates an eddy current detection probe including
an array of drive coils and detection loops according to another
non-limiting embodiment.
[0029] FIG. 9 illustrates an eddy current detection probe including
an array of solid state sensors with a single drive coil, according
to another non-limiting embodiment.
[0030] FIG. 10 illustrates a non-limiting example of a block
diagram of a bucking circuit that may be used with sensors of the
types described herein.
[0031] FIG. 11 illustrates an example of a detection signal
signature according to embodiments described herein.
[0032] FIG. 12 is an example of the configuration of a crack or
other defect relative to the drive coil of an eddy current
detection probe as described herein that gives rise to a detection
signature including four peaks of the type illustrated in FIG.
10.
[0033] FIG. 13 illustrates an eddy current detection probe for
determining the thickness of a surface layer of different
electromagnetic properties.
DETAILED DESCRIPTION
[0034] The inventors have appreciated that conventional eddy
current probes are not without drawbacks in at least some operating
scenarios. For example, a problem with conventional coil-based eddy
current probes is that, as described above, the orientation of the
detection coil is such that the detection coil is sensitive to the
magnetic fields directly generated by the drive coil. Because the
magnetic fields generated by the drive coil are typically much
larger than the magnetic field disturbances associated with cracks
(or other defects) in a material under test, conventional
coil-based eddy current probes may have difficulty in detecting
such cracks (or other defects). Also, in the case of conventional
single-coil based eddy current probes (i.e., an eddy current probe
using a combination drive/detection coil), the design parameters of
the detection circuit cannot be optimized independently of the
parameters of the drive circuit.
[0035] The use of an AMR sensor may address some problems with
respect to conventional coil-based eddy current probes. Namely, the
AMR sensor can be oriented to be insensitive to the magnetic field
produced by the drive coil. However, the inventors have appreciated
the signal from magnetoresistive sensors, including AMR sensors, is
frequency independent, while the signal from coil sensors increases
linearly with frequency. Thus at higher frequencies (e.g. above 1
MHz) used, for example, for analyzing thin structures or materials
with low conductivities, the signal from the coil based sensors may
be substantially higher than the signal from an AMR sensor.
Conversely, at low frequencies, used for example to examine
materials with high conductivities or defects buried more deeply
beneath the surface, the AMR sensor signal may be higher than that
from a coil based detector.
[0036] According to one aspect of the invention, a hybrid eddy
current detection probe is provided that includes both a solid
state sensor (e.g., an AMR sensor) and a detection loop. The solid
state sensor may be configured to be responsive to specific
components of a magnetic field produced by eddy currents, which in
some situations may result from an incident magnetic field applied
by the probe. The detection loop may be responsive to components of
magnetic flux associated with the magnetic field produced by the
eddy currents. Thus, each of the solid state sensor and the
detection loop may provide an output signal (also referred to
herein as a "detection signal") indicative of the magnetic field
produced by the eddy currents. In some non-limiting embodiments, a
combined output signal including a contribution from the solid
state sensor and a contribution from the detection loop may be
provided by the probe. The sensor may be configured to be
especially sensitive to magnetic field components associated with
perturbations to the eddy current flow paths associated with cracks
or other irregularities in the material, e.g. corrosion damage,
inclusions, surface roughness, or other flaws.
[0037] In some aspects of the invention, an eddy current detection
probe is provided that includes a drive coil and a detection loop
having a sensitive axis that is not parallel to the direction of
the magnetic field generated by the drive coil. In some
non-limiting embodiments, the detection loop may be oriented
substantially perpendicular to the drive coil such that the
sensitive axis of the detection loop may be substantially
perpendicular to the direction of a magnetic field generated by the
drive coil and to the induced magnetic field generated in a uniform
substrate in the absence of cracks or other irregularities, and
would thus only be sensitive to fields disturbances arising from
such cracks or irregularities. However, not all embodiments are
limited in this respect, as the drive coil and detection loop may
have any suitable non-aligned orientation relative to each other.
In some embodiments (e.g., when the detection loop is oriented
substantially perpendicular to the drive coil), the detection loop
has a small height (e.g., less than 2.5 mm, less than 1.0 mm or
less than 0.5 mm) which corresponds to the dimension of the loop
that is substantially perpendicular to the drive coil. Such a
construction enables the probe to have a small size which can
enhance performance as described further below.
[0038] According to another aspect of the invention, a method of
operating an eddy current detection probe is provided. The method
comprises exciting a drive coil of the eddy current detection probe
with an alternating current (AC) signal having a high frequency
(e.g., greater than approximately 1 MHz) and applying to a material
under test an incident magnetic field generated by the drive coil
as a result of exciting the drive coil. The method further
comprises detecting an induced magnetic field from the material
under test using a conductive detection loop oriented substantially
perpendicular to the drive coil. The use of the high frequency
excitation signal may facilitate detection of cracks in low
conductivity materials under test and may increase an output signal
of the detection loop compared to if lower frequencies of the AC
excitation signal were used.
[0039] According to another aspect of the present invention, a
method of manufacturing an eddy current detection probe comprises
manufacturing a drive coil and/or detection loop of the probe on a
multi-layered substrate. According to one embodiment, conductive
traces on various ones of the multiple layers of the substrate are
connected using conductive interconnects to form the drive coil
and/or detection loop.
[0040] The above-described aspects, as well as additional aspects,
are now described in greater detail. These aspects can be used
independently, all together, or in any combination of two or
more.
[0041] As mentioned, according to one aspect, an eddy current
detection probe comprises both a solid state sensor and a detection
loop, and thus may be considered a hybrid eddy current detection
probe. FIG. 3 illustrates a side view of a non-limiting example. As
shown, the eddy current detection probe 300 includes both a solid
state sensor 302 and a detection loop 304. The probe 300 further
includes a substrate 306 on which the solid state sensor is
disposed, and a drive coil 308.
[0042] The solid state sensor 302 may be any suitable solid state
sensor for detecting magnetic fields. According to one non-limiting
embodiment, the solid state sensor is an AMR sensor. According to
one non-limiting embodiment, the solid state sensor 302 is a GMR
sensor. Other types of solid state sensors may also be used, as AMR
sensors and GMR sensors represent non-limiting examples. Optionally
the solid state sensor may be biased so as to influence the
direction of sensitivity and/or to linearize its output signal.
Biasing may be provided using permanent magnets, conductive
"barberpole stripes" or by any other means known in the art. In
some embodiments, the solid state sensor (e.g., AMR sensor) may be
configured to be sensitive to magnetic fields along only one
axis.
[0043] As shown, the solid state sensor 302 may be disposed on a
substrate 306. In some embodiments, the substrate 306 may have a
substantially planar surface and the solid state sensor 302 may be
disposed on the substantially planar surface. It should be
understood that other embodiments may include non-planar substrate
surfaces. The substrate may be formed of a material that is not
magnetically reactive, so as to minimize the impact of the
substrate on the probe function. In some embodiments, the substrate
306 may be formed of silicon, though other suitable substrate
material may also be used.
[0044] The inset of FIG. 3 illustrates a non-limiting example of a
top view of a configuration of the solid state sensor 302. As
shown, the solid state sensor may comprise a strip 310 between pads
312a and 312b, and thus may be termed a "strip sensor". The sensor
may be disposed on the substrate 306. In one such embodiment, the
solid state sensor 302 is an AMR sensor, though not all embodiments
are limited to using AMR sensors as the solid state sensor.
[0045] The solid state sensor 302 may be configured in any suitable
manner for detecting magnetic field components arising from cracks
or irregularities in the substrate, i.e., perturbed magnetic
fields. For example, the solid state sensor 302 may be arranged to
have one or more of its sensitive axes in any suitable relationship
with respect to the anticipated direction of the perturbed magnetic
fields. As a non-limiting example, the eddy current detection probe
300 may be arranged with the drive coil 308 substantially parallel
to the surface of a material under test. In such an arrangement,
the magnetic field generated by the drive coil and the magnetic
field induced in a uniform material may both be incident
substantially perpendicular to the surface of the material under
test (i.e., in the z-direction in FIG. 3). Cracks (or other types
of defects) in the material under test may be expected to generate
induced magnetic fields having components in the x-y plane of FIG.
3, and thus the solid state sensor 302 may be configured to have
one or more (if there is more than one) sensitive axis arranged in
the x-y plane.
[0046] In some embodiments, the solid state sensor 302 may be
configured to minimize detection of magnetic fields generated by
the drive coil (i.e., incident magnetic fields). In this manner,
the ability of the solid state sensor to detect perturbed magnetic
fields may be increased. According to one such embodiment, the
solid state sensor is dimensioned to have only one sensitive axis,
thus allowing for the sensor to more easily be oriented to be
insensitive to the magnetic fields generated by the drive coil (as
opposed to if the sensor had two or more sensitive axes, which
would make it more difficult to arrange the sensor such that each
sensitive axis is insensitive to the magnetic fields generated by
the drive coil). As an example, the solid state sensor 302 may be
an AMR sensor and may be dimensioned to have only one sensitive
axis. Such dimensioning may involve making the AMR sensor
sufficiently small in two of the three dimensions (e.g.,
sufficiently narrow in the y-direction of FIG. 3 and sufficiently
thin in the z-direction of FIG. 3) to result in only one of the
dimensions (e.g., that along the x-axis of FIG. 3) being sensitive
to magnetic fields.
[0047] However, it should be appreciated that eddy current
detection probes of the type illustrated in FIG. 3 are not limited
to having a solid state sensor with only one sensitive axis and are
not limited to dimensioning the solid state sensor in any
particular manner, as the example given above is merely provided
for purposes of illustration. Furthermore, at the center of the
coil both the incident magnetic field and the magnetic field due to
eddy currents induced in a uniform material are exactly
perpendicular to the surface, allowing a sensor with more than one
in-plane sensitive axis in the plane to be placed there without
sensing the incident magnetic field.
[0048] The solid state sensor 302 may be configured to minimize its
sensitivity to the magnetic fields generated by the drive coil 308
by positioning it suitably relative to the drive coil and by
orienting its sensitive axis (or axes) appropriately for its
position relative to the drive coil. For example, according to one
non-limiting embodiment, the solid state sensor 302 may be
substantially centered within the drive coil 308 where the in-plane
components of the magnetic field generated by the drive coil are
low, and may be centered on the principal axis of the drive coil so
that any residual in-plane components of the magnetic field
generated by the drive coil average to zero due to symmetry over
the volume of the sensor. By arranging the solid state sensor in
this manner, the majority or entirety of any output signal
("detection signal") from the solid state sensor may be
attributable to perturbed magnetic fields from a non-circular
distribution of eddy currents in a material under test and not to
the incident magnetic field generated by the drive coil 308.
However, not all embodiments are limited to utilizing this
particular positioning of the solid state sensor relative to the
drive coil. There may be other positions relative to the drive coil
where one or more components of the incident field are low and the
sensor 302 may be placed there by a suitable choice of orientation
of its sensitive direction.
[0049] In one such embodiment, the solid state sensor may be
positioned with its sensitive axis tangential to the geometry of
the drive coil. For example, in those embodiments in which the
solid state sensor is sensitive along only one of its axes, the
sensor may be positioned with its insensitive in-plane (i.e., in
the plane of the drive coil) direction coincident with a diameter
of a circular drive coil and its out-of-plane direction parallel to
the principal axis of the drive coil (i.e., in the z-direction in
FIG. 3). FIG. 4 illustrates a top-down view of a non-limiting
example, showing four different potential locations P.sub.1-P.sub.4
of the solid state sensor oriented with its sensitive axis
tangential to the geometry of the drive coil. Note that the
embodiment is not limited in this respect and the probe may
comprise any desired number of sensors (i.e. 1, 2, 4 or more
sensors may be placed along diameters of the coil with their
sensitive axis aligned in the tangential direction). Assuming that
the in-plane (i.e., in the page of FIG. 4) length of the sensor
represents the insensitive in-plane axis of the sensor and that the
arrow over each sensor in FIG. 4 identifies the sensitive axis of
the sensor, it is seen that for each of positions P.sub.1-P.sub.4
the in-plane insensitive direction is coincident with a diameter of
the drive coil (represented by the dashed lines) and that the
sensitive axis is tangential to the drive coil. Although the
in-plane magnetic field due to the incident field and the induced
field in a uniform material may be significant at these locations,
its orientation is purely radial, and the tangential field is zero
unless cracks or other irregularities in the surface disrupt the
eddy currents and give rise to a perturbed induced magnetic
field.
[0050] It should be appreciated that in the configuration of FIG. 4
the sensor 302 (in any of the four illustrated possible locations
P.sub.1-P.sub.4) is de-centered, i.e., not centered within the
drive coil. By positioning the sensor more closely to the drive
coil perimeter (i.e., by not centering the sensor), the sensitivity
of the sensor to small cracks or irregularities may be increased.
It should be appreciated that analogous positions also exist
outside the coil, illustrated as positions P.sub.5-P.sub.8. It
should also be appreciated that similar positions (i.e. positions
where the local direct and induced field is zero along one
direction, and non-zero in the presence of a crack or flaw, and in
which a sensor may be placed with its sensitive axis aligned with
this direction) may be identified for drive coils that are square,
rectangular, or of other convenient shapes. It should furthermore
be appreciated that this concept may also be utilized with a linear
drive wire as opposed to a drive coil, for example to allow for
close spacing of the solid state sensor to the drive source (drive
coil or linear drive wire).
[0051] The detection loop 304 may take any suitable configuration.
According to the non-limiting embodiment illustrated, the detection
loop 304 may be formed by lead lines connected to the solid state
sensor 302 (which may be referred to as "pickup leads") to read out
a signal from the solid state sensor. However, not all embodiments
are limited to this configuration, as other embodiments allow for
the detection loop 304 to be electrically disconnected from the
solid state sensor 302. Similarly, the detection loop 304 may be a
coil in some embodiments, including any suitable number of turns,
and may be formed of any suitable material (e.g., a conducting
material such as a metal (e.g., copper as a non-limiting example),
a conducting polymer, or other conducting material). Using a larger
number of turns for the detection loop may increase the signal
generated by the detection loop. As shown, the detection loop 304
may be arranged around (e.g., wrapped around), or at least
partially around, the substrate 306 on which the solid state sensor
is disposed. As will be described further below, the detection loop
may be small in some embodiments, and thus may be a microscopic
loop in some embodiments. For example, the detection loop may have
a small height h (e.g., the dimension substantially parallel to the
axis of the drive coil) and/or the detection loop may enclose a
small area. In embodiments in which the detection loop is arranged
around the substrate, the height of the detection loop may be
determined by the thickness of the substrate and, thus, the area
enclosed by the detection loop is also determined, in part, by the
thickness of the substrate.
[0052] In one embodiment, the detection loop 304 may be formed by
interconnecting metal traces on different layers of a printed
circuit board using vias or in any other suitable manner, as
described further below. In such an embodiment, the drive coil may
also be formed on the PCB, though not all embodiments in which the
detection loop is formed on a PCB are limited in this respect. In
those embodiments in which the detection loop and/or drive coil are
formed on a PCB, the substrate 306 may be omitted (or,
alternatively, the PCB may be considered to be the substrate
306).
[0053] As with the solid state sensor 302, the detection loop 304
may be designed to be insensitive to the magnetic field created by
the drive coil, thus increasing the ability of the detection loop
to detect perturbations in the induced magnetic fields associated
with eddy currents in a material under test. For example, as can be
seen from the embodiment of FIG. 3, the principal axis of detection
loop 304 may be substantially perpendicular to the principal axis
of the drive coil (e.g., the principal axis of the detection loop
may be in the x-direction while the principal axis of the drive
coil is in the z-direction). Other configurations are possible,
however, as a substantially perpendicular orientation of the
detection loop relative to the drive coil is one non-limiting
example (e.g., the two may have principal axes oriented at 20
degrees relative to each other, 30 degrees relative to each other,
45 degrees relative to each, 60 degrees relative to the each other,
75 degrees relative to each other, or any other suitable angle
(e.g., any suitable angle between approximately 10 degrees and 170
degrees)).
[0054] For example, according to one embodiment, the detection loop
304 may be placed along a coil diameter with its principal axis
(and therefore its sensitive direction) oriented along a
substantially tangential direction, analogous to the positioning of
hybrid sensors in FIG. 4 as discussed above.
[0055] The drive coil may be any suitable drive coil, such as those
conventionally used in eddy current probes of the types shown in
FIGS. 1 and 2. The drive coil may be a pancake coil in some
embodiments. The drive coil may have any suitable shape, including
circular (e.g., as shown in FIG. 4), square, and rectangular, among
others.
[0056] In the non-limiting example of FIG. 3, the drive coil may be
larger than the substrate 306 and thus may be stabilized on its own
support structure. However, not all embodiments are limited in this
respect. For example, according to some embodiments, the drive coil
may be disposed on or in the same substrate 306 as that on or in
which the solid state sensor 302 is disposed. For example, the
drive coil may be formed on a top surface of the substrate and/or
on the bottom surface of the substrate (e.g., See FIG. 6).
[0057] One or more of the following benefits, among others, may be
realized by the configuration of an eddy current probe of the type
illustrated in FIG. 3. First, the orientation of the drive coil to
be substantially parallel to the surface of the material under test
when in use may maximize the coupling between the drive coil and
the material under test, thus creating large eddy currents.
Secondly, the orientation of the detection loop to be substantially
perpendicular to the surface of the material under test when in use
may maximize the sensitivity of the detection loop to the magnetic
field components parallel to the surface caused by cracks or other
flaws. Thirdly, the relative orientation of the detection loop
perpendicular to the drive coil may minimize the direct coupling
between the drive coil and the detection loop. Fourth, the
orientation of the detection loop to detect the magnetic flux
associated with eddy currents in the material under test may
complement detection of the defect by the solid state sensor,
especially at high frequencies. In that manner, a single probe may
be utilized to cover a wide range of frequencies, with the signal
from the solid state sensor (AMR, GMR, or other) dominating at
frequencies below a transition frequency and the signal from the
detection loop dominating above the transition frequency. The
transition frequency may be design dependent and may occur around 1
MHz. Thus, probes of the types described herein may operate with
large frequency bandwidths.
[0058] Eddy current detection probes of the type illustrated in
FIG. 3 can be of any suitable size. In some embodiments, the eddy
current detection probes may be small. A non-limiting example of
dimensions of some of the components is now given. As shown in FIG.
3, in one non-limiting embodiment, the substrate 306 may be less
than or approximately equal to 1000 microns wide. The substrate 306
may have a thickness of less than or approximately equal to 500
microns. The solid state sensor may have a strip 310 that is less
than or approximately equal to 250 microns long (e.g., in the
x-direction of FIG. 3), a width (e.g., in the y-direction of the
side view of FIG. 3) that is less than or approximately equal to 8
microns, and a thickness (e.g., in the y-direction of the side view
of FIG. 3) that is less than or approximately equal to 30
nanometers. Such dimensioning of the solid state sensor may result
in the solid state sensor being sensitive to magnetic fields only
along one axis (e.g., only along the y-direction in the side view
of FIG. 3). Other dimensions for the solid state sensor are also
possible, as the numbers listed above represent non-limiting
examples.
[0059] The detection loop may have any suitable size, and in some
embodiments may be a microscopic loop. According to one embodiment,
the detection loop 304 may enclose an area of less than or
approximately equal to 1000 microns by less than or approximately
equal to 500 microns, an area of less than or approximately equal
to 0.50 mm.sup.2; according to another embodiment, the detection
loop 304 may enclose an area of less than or approximately equal to
0.20 mm.sup.2. According to some embodiments, the detection loop
304 has a small height (e.g., dimension h in FIG. 3). For example,
the height of the detection loop may be less than or approximately
equal to 250 microns; and, in some embodiments, less than or
approximately equal to 150 microns.
[0060] According to one embodiment, probes of the type illustrated
in FIG. 3 and utilizing a detection loop in combination with a
solid state sensor may be formed on a printed circuit board (PCB),
flexible printed circuit board or other three dimensional
membrane.
[0061] In operation, the eddy current detection probe 300 may be
placed in proximity to a material under test to test for the
presence of cracks or other defects. The drive coil may be excited
with an AC current, thus generating an incident magnetic field
which impinges upon the material under test. The excitation
frequency may have a broad range and in some instances may exceed 1
MHz although not all embodiments are limited in this respect. Eddy
currents may thus be generated in the material under test, and the
eddy currents may generate induced magnetic fields; cracks or other
irregularities in the surface may generate magnetic field
components parallel to the surface of the material under test which
are detected by the combination of the solid state sensor and the
detection loop.
[0062] In those embodiments in which the solid state sensor is an
AMR sensor, the resistance of the AMR sensor may vary in dependence
on the magnetic field where the sensor is located. Such variations
in the resistance value may be detected by changes in a signal read
from lead lines of the AMR sensor (e.g., from the detection loop in
those embodiments in which the detection loop comprises the lead
lines). In addition, the detection loop may enclose some of the
magnetic flux associated with the induced magnetic fields. Changes
in the magnetic flux due to the eddy currents induce a voltage in
the detection loop which may be detected by suitable readout
electronics (not shown). Thus, in the example of FIG. 3, both the
solid state sensor and the detection loop contribute to the total
detection signal output by the lead lines forming the detection
loop and therefore both contribute to detection of magnetic fields
associated with eddy currents in the material under test.
[0063] However, as previously mentioned, it should be appreciated
that not all embodiments are limited to the detection loop being
electrically connected to the solid state sensor as lead lines of
the solid state sensor. Rather, in some embodiments the detection
loop may be electrically distinct from the solid state sensor
(e.g., the solid state sensor may have its own dedicated lead
lines). In such an embodiment, distinct signals may be output by
the solid state sensor and the detection loop and may be processed
in any suitable manner (e.g., processed individually, combined and
processed, etc.) to assess whether the induced magnetic fields
indicate the presence of a crack or other defect in a material
under test.
[0064] The relative magnitude of a signal output by the solid state
sensor 302 compared to a signal output by the detection loop 304,
or the relative magnitude of contributions to a single signal
attributable to the solid state sensor and the detection loop
(i.e., in those embodiments in which a single signal is provided by
the solid state sensor and detection loop), may depend on various
factors. To first order, a solid state (AMR, GMR, or other) sensor
is sensitive to the field H, and the detection loop to the product
of frequency f, field H, the number of turns in the detection loop,
and the area enclosed by the loop A. Thus, in those embodiments in
which the solid state sensor 302 is an AMR sensor or GMR sensor,
the contribution of the detection loop may become significant and
in fact dominant over that of the solid state sensor at drive coil
frequencies above a transition frequency which depends on the
specific design of the sensor and the detection loop. For example,
the contribution from the detection loop may become substantial
above 500 kHz or at frequencies greater than approximately 1
MHz.
[0065] The inventors have realized that some applications do not
require the use of a solid state sensor. A non-limiting example is
illustrated in FIG. 5. As shown, the eddy current detection probe
500 includes the detection loop 304, substrate 306, and drive coil
308 of FIG. 3. However, the eddy current detection probe 500
differs from the eddy current detection probe 300 of FIG. 3 in that
no solid state sensor is included. Such a probe design may be
suitable in situations in which the output signal of the detection
loop is sufficient to exceed any anticipated noise level, such that
the solid state sensor 302 of FIG. 3 is not needed to overcome the
noise level. Additionally, the substrate 306 may optionally be
omitted as it may not be needed to support the detection loop 304
(e.g., the detection loop 304 may be formed without enclosing any
substrate).
[0066] According to one aspect, an eddy current detection probe
includes a drive coil and a detection loop, with the detection loop
having a principal sensitive axis (also referred to herein as the
"axis of sensitivity") that is not aligned with the principal axis
of the drive coil. As in the case of the hybrid sensor discussed
above, the detection loop and drive coil may be oriented relative
to each other in any suitable manner to: (a) minimize detection of
the incident magnetic field by the detection loop; and/or (b)
increase the sensitivity of the detection loop to perturbations in
the induced magnetic fields associated with cracks or other defects
in a material under test.
[0067] The detection loop may have a principal sensitive axis
oriented to be non-parallel with the principal axis of the drive
coil 308. For example, in the non-limiting embodiment of FIG. 5,
the drive coil 308 may have a principal axis oriented in the
z-direction. The detection loop may have a principal axis oriented
in any direction other than the z-direction. By configuring the
drive coil and detection loop such that their principal axes are
not parallel to each other, the sensitivity of the detection loop
to magnetic fields generated by the drive coil may be minimized and
the sensitivity of the detection loop to perturbations in the
induced magnetic fields due to defects such as cracks in a material
under test may be increased.
[0068] According to one non-limiting embodiment, the detection loop
304 is substantially perpendicular to the drive coil 308, in that
the principal axis of the detection loop is perpendicular to the
principal axis of the drive coil. In some embodiments, the
principal axis of the detection loop may be oriented at an angle of
between 10 degrees and 170 degrees (e.g., 30 degrees, 45 degrees,
60 degrees, 75 degrees, 90 degrees, 105 degrees, 120 degrees,
between approximately 30 degrees and 90 degrees, between
approximately 60 degrees and 90 degrees, or any other suitable
angle) relative to a principal axis of the drive coil.
[0069] Referring to FIG. 5, a substantially perpendicular relative
orientation of the drive coil and detection loop is shown, as the
principal axis of the detection loop may be in the y-direction and
the principal axis of the drive coil may be in the z-direction.
Such a perpendicular configuration may be beneficial, for example
by maximizing the sensitivity of the detection loop to
perturbations in the magnetic fields generated by cracks or other
defects in a material under test parallel to the drive coil and by
minimizing the sensitivity of the detection loop to the magnetic
fields generated by the drive coil.
[0070] According to one embodiment, the detection loop may surround
the drive coil or wire. FIG. 6 illustrates one non-limiting case.
The figure shows a cross section taken in the plane of the
detection loop 304. The drive wire 308 passes through the center of
the area enclosed by the detection loop. This geometry is a
limiting case of the geometry illustrated in FIG. 4.
[0071] FIG. 7 illustrates a variation on the eddy current detection
probe of FIG. 5. As shown, the eddy current detection probe 600 is
placed in proximity to a material under test 602 including a defect
604. The probe 600 includes a membrane 606 (which may also be
thought of as a substrate) formed by a sandwich structure of
conductive layers and resistive layers. A drive coil 608 is placed
on a top surface of the membrane 606 and a drive coil 610 is placed
on a bottom surface of the membrane 606. A detection loop 612 is
oriented substantially vertically within the membrane 606 and thus
is substantially perpendicular to the drive coils 608 and 610.
[0072] As can be seen in FIG. 6, according to one embodiment a side
614 of the detection loop 612 may be in substantially the same
plane as the drive coil. For instance, as shown, the membrane 606
may have a substantially planar top surface on which the drive coil
608 is disposed. The side 614 of the detection loop 612 may be
disposed on the top surface as well, thus making the side 614
co-planar with the drive coil 608. Other configurations are also
possible, however, as the shown configuration is a non-limiting
example.
[0073] The probe 300 of FIG. 5 may be operated at high frequencies
in some situations, which may, for example, increase the detection
signal from detection loop 304. As mentioned previously, it may be
desirable to probe a material under test using high frequency
incident magnetic fields, for example if the material under test is
a low conductivity material. Generation of a high frequency
incident magnetic field may be accomplished by exciting the drive
coil 308 with a high frequency alternating current (AC) signal. The
resulting induced magnetic fields in the material under test
detected by the detection loop 304 may therefore also be high
frequency fields. Detection loop 308 may output larger signals in
response to detection of higher frequency fields, such that use of
high frequency magnetic fields may facilitate detection of certain
types of flaws.
[0074] According to one embodiment, an eddy current detection probe
of the type illustrated in FIG. 5 may be excited with AC drive
signals between approximately 500 kHz and 10 MHz, or of any other
suitable value. However, it should be understood that lower
frequencies (e.g., frequencies between 100 Hz and 500 KHz) may also
be used in certain embodiments. One feature of the eddy current
detection probes described herein is that they are capable of being
excited with AC drive signals over a broad range of frequencies.
For example, the probe may be excited with AC drive signals between
100 Hz and 10 MHz; in some embodiments, between 1 KHz and 10 MHz,
and in some embodiments, between 10 KHz and 10 MHz. The frequency
used to excite the probe may depend on the application in which it
is used. Operation over such a broad frequency range enables the
eddy current detection probes to be used in a wide variety of
applications. Furthermore, the method is not limited to using such
frequencies for any particular purposes (e.g., detecting cracks in
low conductivity materials being one non-limiting example).
Furthermore, it should be appreciated that such methodology may
also be applied to eddy current probes of the type illustrated in
FIG. 3 including a solid state sensor in addition to a detection
loop.
[0075] According to another aspect, an array of probes of the types
previously described herein may be formed. FIG. 8 illustrates a
non-limiting example.
[0076] As shown, the array 700 may include multiple drive
coil-detection loop combinations 702. In the embodiment shown,
eight such combinations of the type of probe illustrated in FIG. 7
are included (i.e., eight combinations of drive coils and detection
loops). In this manner, a larger area of the material under test
may be investigated at a single time. It should be understood that
other combinations are also possible. While the array 700
illustrates drive coils and detection loops of the type illustrated
in FIG. 7, it should be appreciated that the aspects described
herein relating to arrays of eddy current detection probes are not
limited in this manner. For example, an array of the type
illustrated in FIG. 8 may be formed using multiple probes of the
type illustrated in FIG. 3 (i.e., multiple probes each including a
solid state sensor and detection loop). Other configurations are
also possible.
[0077] The drive coils and detection loops of the array 700 may be
electrically connected or separate. Namely, the drive coils may be
driven together or separately. If driven together (and therefore
not sequentially) the drive coils may be electrically connected via
a single set of lead lines (e.g., lead lines 704). If driven
separately, separate lead lines may be connected to each of the
drive coils. If driven separately, the drive coils may be driven
simultaneously or at different times (e.g., sequentially, in groups
of two or more at a time, or otherwise time divided). Similarly,
the detection loops may be read out together or separately. In one
example the detection loops may be read substantially
simultaneously in contrast to conventional systems in which, due to
the mutual inductance of the sensors, sequential excitation of
drive coils and sequential reading of sensors is needed. In the
situation in which the detection loops are read out together, they
may be electrically connected and read by a single set of lead
lines (e.g., lead lines 706). If the detection loops are read out
separately, each loop may be connected to its own set of lead
lines, and the loops may be read out at any suitable relative
timing (e.g., simultaneously, sequentially, in groups of two or
more at a time, or otherwise time divided, etc.). Thus, it should
be appreciated that the array of drive coils and detection loops
may be operated in any suitable manner.
[0078] Moreover, according to one embodiment, an array of solid
state sensors and/or detection loops may be used with a single
drive coil. An example is illustrated in FIG. 9. As shown, an eddy
current detection probe 800 may include multiple (i.e., eight in
this non-limiting case) solid state sensors 802 of the type in FIG.
3 in combination with a single drive coil 804. In this example, the
drive coil 804 is rectangular in shape, though other shapes may
alternatively be used. According to one embodiment in which a
single drive "coil" is used with an array of sensors, the drive
"coil" may be a linear drive wire. Since all the sensors operate in
connection with a single drive coil/wire, sequential scanning of
the sensor outputs may be avoided.
[0079] Using one or more of the aspects described above with
respect to arrays of sensors, improved speed in scanning a material
under test may be provided compared to conventional sensors. For
example, the sensors of the array may be configured in a linear or
multidimensional array so that an image of the eddy currents and
defects in a material under test can be created without moving or
scanning the sensors relative to the material under test.
[0080] According to another aspect, a linear drive wire, as opposed
to a drive coil, for generating an incident magnetic field and
therefore the eddy currents in a material under test is provided.
For example, referring again to the non-limiting embodiment of FIG.
9, the drive coil may be formed as a drive wire, for example by
including only one of the sides of the coil illustrated. For
example, the side of the coil 804 under the array of solid state
sensor 802 in the figure may be retained.
[0081] In utilizing eddy current detection probes having solid
state sensors of the types previously illustrated, it may be
beneficial for the solid state sensor to be as close to the drive
coil as possible, since such positioning may result in a stronger
detection signal from the solid state sensor. However, with such
solid state sensors, it is also preferable in some situations, as
previously described, for the solid state sensor to be
approximately centered within the drive coil. Thus, in situations
in which a drive coil has a substantially closed geometry (e.g., a
circular geometry, a rectangular geometry, etc.), a design in which
the solid state sensor is to be centered within the drive coil
places a limitation on how close the solid state sensor can be to
the drive coil, since moving the solid state sensor closer to one
portion of the drive coil may result in de-centering of the solid
state sensor. Thus, competing design constraints may come into
play.
[0082] In view of such competing design constraints, one aspect
provides a linear drive wire rather than a drive coil. In this
manner, a sensor (e.g., a solid state sensor alone, a solid state
sensor in combination with a detection loop having a cross section
perpendicular to the drive wire, or a detection loop alone) may be
positioned more closely to the drive wire than would be possible
using a drive coil, and therefore the sensor may be able to produce
stronger detection signals than would be otherwise possible, thus
allowing for detection of smaller defects. In such a configuration
in which a linear drive wire is used, there is no need to center
the sensor with respect to the drive wire (i.e., the sensor can be
to the right or left of the drive wire or the detection loop may
enclose the drive wire and the sensor will operate
appropriately).
[0083] The eddy current detection probes described herein may be
formed/manufactured in any suitable manner. According to one
aspect, drive coils and/or or detection loops of the types
described herein may be formed on multilayered substrates by
interconnecting traces on various layers of the substrate. For
example, substrate 306 shown in FIGS. 3 and 5 may be a
multi-layered substrate, for instance being a multi-layered printed
circuit board (PCB). Conductive traces on various ones of the
layers may be interconnected using conductive interconnects to form
the drive coil (e.g., in those embodiments in which the drive coil
is formed on substrate 306) and/or the detection loop. For example,
the traces on the surface of a substrate may form the geometry of
the detection loop and/or drive coil in a plane. Multiple layers of
this substrate can form multiple planes, on which many drive coils
and/or detection loop turns and connecting traces can be
constructed. Conductive holes (i.e. "vias") in each layer, or
through all layers, can form the interconnections between detection
loop/drive coils, create multiple turns in the substrate plane, or
form the closed path for loops/coils perpendicular to the substrate
plane. In some embodiments, this may allow for arbitrary structures
in the substrate plane (i.e. circles), but may restrict the
detection loop/drive coil structures to rectangles in the
perpendicular plane, because of the limitation that holes are
typically straight. If the detection loop geometry is much finer
than the driver coil, the conductive layers for the detection loop
can be formed by a different process with thinner conductive
material and finer trace width, spacing, and via size. In at least
some embodiments, the detection loop does not require high current
carrying capacity.
[0084] According to some embodiments, substrates can be made of
almost any non-conductive material that can be drilled and layered
with etchable conductive material. Some non-limiting examples
include FR4 epoxy, polyimide, silicon, ceramic, and Teflon. Highly
conductive materials such as copper, silver, gold, conductive
polymer or graphene may form the detection loops and/or drive coils
in at least some embodiments.
[0085] Thus, it should be appreciated that eddy current probes of
the types described may be formed using printed circuit board
and/or photolithographic techniques (e.g., traces and/or coils may
be defined photolithographically according to standard
microfabrication processing techniques).
[0086] Similarly, a hybrid eddy current detection probe including a
solid state sensor (e.g., an AMR sensor) and a detection loop may
be made by placing the AMR inside a conductive driver coil with
fine placement precision. Hand wound coils present the problem of
having to mechanically align the AMR sensor such that crosstalk is
minimized. If a photolithographically fabricated drive coil is
used, it, the detection loop, and connecting traces that are part
of the hybrid eddy current detection probe may be etched with
standard printed circuit board and/or microfabrication process
steps as part of the sensor fabrication. As an alternative,
connection pads and alignment points can be fabricated together as
part of one multi-step process to facilitate accurate placement of
a separately diced silicon AMR sensor. In this way, a flexible
backing may be used and complex, custom sensor arrays may be made
with standard AMR cells.
[0087] Use of the fabrication techniques described above may allow
for various possible constructions scenarios, some of which have
been previously described. For example, the detection loop(s) may
be fabricated within the drive coil(s) or around the drive coil(s).
Detection loops may be wrapped around a conductor serving as a
drive coil (e.g., when the drive coil is a straight trace, i.e., a
drive wire.) The drive coils and detection loops may take various
suitable shapes, including rectangular, diamond-shaped,
wave-shaped, and parabolic curve shaped, among others. The drive
coils may include any suitable number of turns. Multiple
differential matching detection loops may be fabricated.
[0088] As non-limiting examples, a four turn driver coil can be
constructed with one circular turn on the 1.sup.st (top), 3.sup.rd,
4.sup.th and 6.sup.th layers of a 6-layer process. One or two
substrate layers may separate each coil turn and conductive holes
(vias) may connect the turns in such a way as to create a stepped
helix. Such a geometry may be beneficial for thicker, current
carrying driver coils. Detection loops may be constructed
perpendicular to the layers using the 2.sup.nd and 5.sup.th
conductive layers with vias completing the rectangular loops.
Connecting traces can come inside the driver coil on these layers
to connect the many turn loop to sensing electronics.
[0089] It should be appreciated that the relative sizes of the
drive coils and detection loops are not limiting. In some
embodiments, such as shown in FIGS. 3 and 5, the detection loop may
be positioned "inside" the drive coil, in that the detection loop
may be within the diameter of the drive coil. Alternatively, the
detection loop may have at least one dimension larger than the
drive coil, and the drive coil may be "inside" the detection loop.
Such configurations represent two non-limiting possibilities.
[0090] According to another aspect, any of the eddy current
detection probes previously described may be flexible. For example,
drive coils/wires, solid state sensors (e.g., AMR sensors), and/or
detection loops may be formed on flexible materials such as those
used for flexible circuit boards or other conductive membranes. As
a non-limiting example, the substrate 306 in FIG. 3 may be flexible
and the solid state sensor 302 and drive coil 308 may be disposed
on the flexible substrate. In some embodiments, the flexible
substrate may be curved during use, as described further below. In
another non-limiting example, the substrate 306 may be rigid or
substantially rigid, but may be disposed on a larger flexible
substrate, allowing for wrapping of the flexible substrate around a
material under test (e.g., multiple rigid substrates 306 may be
positioned in an array on a larger flexible substrate). Other
configurations of flexible eddy current detection probes are also
possible.
[0091] The eddy current detection probes may be used to identify
defects in a wide variety of articles. In some embodiments, the
probes are particularly useful in identifying smaller defects
(e.g., defects having a size of less than or approximately equal to
500 micron, less than or approximately equal to 250 micron, or less
than or approximately equal to 100 micron). The probes may also be
used to identify defects that are located close to, or at, the
surface of the article being tested. In general, the articles
tested by the probe may be formed of any material having a suitable
conductivity (e.g., a metal, metal alloy) which enables generation
of eddy currents within the material. Advantageously, the probes
described herein are able to identify defects in relatively low
conductivity metals and metal alloys including titanium and
Inconel.RTM., amongst others. A wide variety of different types of
articles may be tested using the probe. Particularly well-suited
articles include those used in heat exchanger applications and jet
engine applications. The use of flexible eddy current detection
probes may allow for inspection of articles having curved surfaces.
For example, flexible eddy current detection probes may be
positioned against a curved surface (e.g., tubes including heat
exchanger tubes, inside a bore hole, against an airplane wing,
etc.) allowing for analysis of typically difficult to reach parts
of materials under test.
[0092] According to another aspect, a bucking circuit is provided
for operation in connection with eddy current detection probes of
the types previously described (e.g., eddy current detection probes
of the types shown in FIGS. 3, 5, and 7). The drive signals
typically used to drive the drive coil of an eddy current detection
probe are sinusoidal in nature. Thus, the detected signal (or
"detection signal) from a solid state sensor or detection loop of
the types described herein may also be sinusoidal in nature.
Moreover, there may be other sinusoidal signals of the same
frequency present in the system due, for example, to inductive
pickup in the leads, eddy currents in unperturbed material, or
residual coupling between the sensor and/or detection loop and the
drive coil. It should be appreciated that such signals may be
apparent in the output of the detector in the absence of cracks.
The detection signals produced by eddy current detection probes
according to the various aspects described herein may change
amplitude and/or phase when a defect is encountered in a material
under test. Such changes in amplitude and/or phase of the detection
signal may allow for detection of a defect.
[0093] The inventors have appreciated that the deviations from the
quiescent signal when a defect is encountered are typically
relatively small. Thus, the inventors have appreciated that the
deviations from the quiescent signal may be detected by recording a
reference signal in a region where there is no crack or defect and
subtracting an inverted sinusoid equal and opposite to the
reference signal from the detection signal (also referred to herein
as an "output signal"). The inverted sinusoid cancels the
substantially constant (i.e. not due to the crack or defect)
portion of the output signal, thus leaving a substantially null
signal except for deviations from a zero value when a defect is
encountered.
[0094] According to one aspect, a bucking circuit is provided for
performing the signal processing described, namely subtracting an
inverted sinusoidal signal from a detection signal. The detection
signal may be pre-conditioned, for example, by applying a bucking
signal (e.g., at the drive frequency). The bucking signal may
comprise an inverted reference signal. The reference signal may be
obtained by making a reference measurement at a point of the
article being tested known to be free of defects. The reference
signal may be obtained by averaging the respective signals obtained
at several different such points of the article being tested. The
bucking signal can allow more efficient utilization of the bit
depth of the digitizing electronic circuit.
[0095] A non-limiting example of a suitable bucking circuit is
shown in block diagram form in FIG. 10, which is described in the
context of an eddy current detection probe of the type illustrated
in FIG. 3, though it should be appreciated that the bucking circuit
may be used with the other types of eddy current detection probes
described herein as well. As shown, the bucking circuit 900
includes a sine wave generator 902 configured to generate and
provide to the drive coil 308 a sinusoidal drive signal 903. The
solid state sensor 302 (and in some embodiments, the combination of
the solid state sensor 302 and the detection loop 304) generates a
detection signal 904. The detection signal is provided to amplifier
906 which may be any suitable amplifier having any suitable gain.
The amplifier outputs an amplified detection signal 908 to a
combining element 910, e.g. a mixer or a summer. The drive coil
signal from sine wave generator 902 is provided to amplifier 912,
which may be any suitable amplifier with any suitable gain, and
which may in some embodiments be the same type of amplifier as
amplifier 906. The amplifier 912 outputs an amplified signal 914
which is provided to the mixer 910.
[0096] The mixer 910 combines (by summing or by any other suitable
process) the signals 908 and 914 and produces an output signal 916,
which is provided to a lock-in amplifier 918. The lock-in amplifier
918 outputs an in-phase signal 920 and a quadrature phase signal
922 to computer 924 which may process the signals in the manner
described above to generate a signal that reflects only deviations
of the detection signal output by sensor 302 from the sinusoidal
shape of the sinusoidal drive signal.
[0097] While FIG. 10 illustrates a non-limiting example of a
bucking circuit, it should be appreciated that bucking circuits
operating in the manner described may take various
configurations.
[0098] One or more benefits may be realized by using a bucking
circuit of the type described in connection with eddy current
detection probes of the types described herein. First, better
signal-to-noise ratio may be achieved. Secondly, in those
embodiments in which an analog-to-digital converter is used to
convert the analog output signal of the solid state sensor and/or
detection loop into a digital signal, fewer bits may be required
since the detection signal has a substantially zero value except
for those limited portions of the signal corresponding to when a
defect is encountered.
[0099] According to another aspect, a detection signal of the eddy
current detection probes of the types described herein is analyzed
for a particular signature in the image of eddy current produced,
related to the geometry of the probe designs described. An example
signature is four peaks derived from the in-phase and quadrature
signals indicating a particular defect.
[0100] Inventors have appreciated that when defects (e.g., cracks)
become relatively small in size compared to the size of the drive
coil of an eddy current detection probe (e.g., when the length of
the crack becomes smaller than the diameter of the drive coil), the
detection signal of the probe may include four peaks 1010a-1010d,
as shown in FIG. 11, which shows the voltage of the detection
signal as a function of space in an x-y plane assuming that the
surface of the material under test is disposed in the x-y
plane.
[0101] FIG. 12 illustrates the cause of the phenomena giving rise
to a detection signal pattern of the type illustrated in FIG. 11.
In typical operation, the eddy current detection probe, and
therefore the drive coil, may be scanned over the material under
test in increments (e.g., similar to a raster scan). This may
result in the drive coil taking various orientations with respect
to a particular defect during the scanning process (e.g., at one
point during the scan the right side of a drive coil may be over a
particular defect while during a later part of the same scan the
left side of a drive coil may be positioned over the defect). In
situations in which the defect is smaller than the diameter of the
drive coil, a noticeable detection response may be produced only
when the drive coil is at each one of four distinct orientations
relative to the crack (i.e., at only four different stages of the
scanning process). FIG. 12 illustrates an example, in which four
distinct locations of a drive coil 1104 relative to a crack 1102
are shown. It should be appreciated that the drive coil assumes
only one of the four illustrated positions at any given time during
the scanning process. If a noticeable detection response is
generated for each of the four illustrated positions of the drive
coil, then the resulting detection signal pattern shown in FIG. 11
may be seen.
[0102] Thus, according to one aspect, the detection signal
generated by an eddy current detection probe may be analyzed for a
signature, such as that shown in FIG. 11, which may be indicative
of certain types of defects or defects having certain
characteristics (e.g., a certain size, etc.).
[0103] AMR sensors are field sensitive and directionally sensitive,
which makes them suitable for numerous applications in addition to
the crack detection application described above. They also have an
extremely low noise floor and may be made very small. The
directionality of the sensors allows them to be used to detect
small field components in the presence of larger fields, for
example detecting small changes of the induced fields in the
presence of the magnetic field generated directly by the drive
coil, as long as the fields of interest have some component
orthogonal to the large component of the background field.
[0104] It should be understood that for certain applications the
optimum geometries may be different from the geometry that is
described above for the application of detecting cracks. For
example, the inventors have appreciated that AMR based sensors may
be used to fabricate a probe suitable for determining the case
hardening depth of heat treated parts. For some combinations of
material and heat treatment processes, the electrical conductivity
of the hardened layer at the surface of the part may be different
from the electrical conductivity of the unmodified material at the
core. The depth distribution of the eddy currents induced in the
part, and therefore also the induced magnetic field, depend on the
depth profile of the magnetic permeability and electrical
conductivity of the material under test, and therefore the response
of a sensor sensitive to the induced magnetic field can be used to
determine the thickness of the case hardened layer.
[0105] For this application the frequency of operation should be
chosen so that the depth distribution of the eddy currents exceeds
the thickness of the case hardened layer. In practice this may mean
operating at low frequencies (frequencies below 500 kHz, below 100
kHz, or below 50 kHz). At these frequencies the response of the
magnetoresistive sensor (e.g. AMR sensor) may exceed that of a
conductive detection loop. Furthermore, since the response of the
magnetoresistive sensor is not dependent on frequency, the
sensitivity of the probe may also be independent of frequency.
Because of this property, probes based on AMR sensors are suitable
for operation in a mode where the response is sampled at various
frequencies, or over a range of frequencies. In situations where
the thickness of the hardened layer is unknown it may be
advantageous to make measurements at a range of frequencies in
order to adequately characterize the conductivity at different
depths. Thus AMR based probes are particularly appropriate for this
application.
[0106] It should be appreciated that for the application of depth
profiling, the material to be tested may be uniform, and therefore
the eddy current distribution may have the same symmetry as the
drive coil. Thus the induced magnetic field will have components in
the vertical and radial directions and not necessarily in the
tangential direction. The magnetic field generated by the driving
current in the drive coil will have components in the same
directions, however the relative magnitude of these various
magnetic field components will vary depending on the location of
the sensor, the direction of sensitivity, and the depth profile of
the electrical conductivity and magnetic permeability of the part.
This is in contrast to the situation described above for detection
of cracks and other flaws, where the in-plane symmetry of the eddy
current paths is disturbed and magnetic field components are
generated which may not be present in the absence of cracks, for
example tangential magnetic field components.
[0107] Thus for profiling the depth dependence of the
electromagnetic properties of the material, the optimum geometry of
the detector relative to the coil can be different from that of the
eddy current crack detector described previously. In this case, the
sensor would be positioned in a location at which at least one of
the magnetic field components has a strong dependence on the depth
distribution of the eddy currents. In some cases, this position of
maximum sensitivity may be outside the perimeter of the drive coil.
In some cases, it may be desirable to place the sensor at a
specific height relative to the drive coil, which may be at its
center, at its lower surface, displaced toward the part, or at any
other height which may be chosen for reasons of sensitivity or
convenient fabrication.
[0108] The axis of maximum sensitivity of the sensor may be placed
in the plane parallel to the surface of the part, in which case it
should be aligned in the radial direction, perpendicular to the
plane, or at any appropriate angle to the plane in order to
maximize the sensitivity of the detector to the depth distribution
of the eddy currents. In some cases, it may be expedient to align
the sensitive axis in the plane parallel to the surface for reasons
of manufacturing and/or reproducibility of results, even if such
placement does not maximize the sensitivity of the probe to the
eddy current depth distribution.
[0109] In some embodiments, the drive coil is configured to be
positioned proximate a surface of the article being tested such
that an axis of the drive coil is perpendicular to the surface of
the article. In some embodiments, the magnetoresistive sensor (e.g.
AMR sensor) is constructed and aligned to respond to the electrical
conductivity and/or magnetic permeability differences between the
material in a layer at the surface of the article and the material
in the interior of the article. In some cases, for example, the
differences between the electrical conductivity and/or magnetic
permeability in a layer at the surface of the article and the
material in the interior of the article are due to case hardening
of the article; and, in some cases, the differences are due to a
coating being applied to the article.
[0110] In some embodiments, the drive coil is disposed on a
substantially planar surface; and in some embodiments, the sensor
may be disposed on the same substantially planar surface as the
drive coil. For example, in some cases, the sensor is positioned
outside the diameter of the coil.
[0111] FIG. 13 shows a schematic representation of a probe that may
be used for depth profiling. A drive coil 308 in close proximity to
the material to be tested has an axis substantially perpendicular
to the surface of the part under test. An oscillating electric
current is passed through the drive coil, which induces a magnetic
field in the part under test 1310. The electromagnetic properties
of the part vary with depth, for example due to a case hardened
layer 1311 which has different electrical conductivity and/or
magnetic permeability than the core material 1312. The distribution
of the eddy current density in the part depends on the depth
profile of the electromagnetic material properties, for example on
the thickness h of the case hardened layer. This eddy current
distribution in turn gives rise to an induced magnetic field, which
alters the magnetic field in the vicinity of the coil. A
magnetoresistive sensor 1320 is positioned in a location and at an
orientation chosen to maximize the sensitivity of the probe to the
depth of the case hardened layer.
[0112] Optionally, multiple sensors may be disposed at different
positions relative to the coil to maximize sensitivity, reduce
noise, or for redundancy.
[0113] The position and orientation of the sensor, the size of the
coil, and frequency of operation may be chosen using a mathematical
model of the system, which may be analytical or numerical (e.g.
using finite element methods). Mathematical models of the system
may be used to fit the data obtained as a function of frequency in
order to determine the thickness of the case hardened layer from
the experimental results. Alternately, the probe may be calibrated
against samples with known surface layer thickness.
[0114] It should be understood that the configuration used for case
hardened layered depth may be useful in determining the depth of
other interfaces substantially parallel to the surface of the part,
including corroded layers, conductive coatings, or similar surface
layers.
[0115] Having thus described several aspects, it is to be
appreciated various alterations, modifications, and improvements
will readily occur to those skilled in the art. Such alterations,
modifications, and improvements are intended to be part of this
disclosure, and are intended to be within the spirit and scope of
the aspects of the invention. Accordingly, the foregoing
description and drawings are by way of example only.
* * * * *